The present invention relates to a surface inspection method and a surface inspection apparatus for inspecting the external appearance of an article and, more particularly, to a surface inspection method and a surface inspection apparatus for inspecting for defects on the surface of an article, which is almost flat and continuous but has non-flat parts with small convex shapes and concave shapes, a surface inspection method and a surface inspection apparatus for precisely setting an inspection area to be inspected, and a surface inspection method and a surface inspection apparatus for accurately judging whether a surface is defective.
Conventionally, defects such as flaws on the external surface of an article were inspected visually but recently, defects are inspected by digitally processing images of a surface taken by a video camera. Particularly, in the semiconductor manufacturing process, external appearance inspection apparatuses are widely used to inspect defects of a fine pattern formed on a semiconductor wafer, contributing to the improvement of the yield. In the steel industry etc., inspection apparatuses are used to inspect defects such as flaws on the surface of steel plates conveyed continuously. In contrast to this, the surface inspection method and the surface inspection apparatus according to the present invention are not used in the semiconductor manufacturing process or in the steel industry, but are used to inspect defects such as flaws on the surface of a molded or machined article.
When a surface of an article, which is almost flat, is illuminated, a defective part can be detected because the defective part such as a flaw reflects light in a manner different from the way a normal part does. A surface inspection apparatus for inspecting an article surface comprises an illuminator for illuminating an article surface, an image pick-up device such as a video camera for taking the picture of an article surface, and a processing device for detecting a defective part by digitally processing the image of an article surface and judging whether the article is a conforming one. The processing device is composed of a computer system, which detects a defective part by comparing the image data converted into multi-valued digital data from an image signal with a threshold value set in advance. For example, when a video camera captures light regularly reflected by an article surface, the luminance of a normally flat part is high but the luminance of a defective part such as a flaw is low, therefore, a threshold value lower than a value corresponding to the luminance of the normally flat part is set, and a part, the luminance of which is lower than the threshold value, is judged to be a defective part. If a higher threshold value is set, the number of parts judged to be defective increases and if a lower threshold value is set, the number of parts judged to be defective decreases.
As described above, when an article surface is illuminated, a defective part reflects light in a different manner from that a normal part does, but the difference in the manners light is reflected depends largely on the illumination method. Because of this, various illuminators for surface inspection apparatuses have been proposed hitherto.
Japanese Unexamined Utility Model Application No. 6-76849 has described a configuration in which LED elements are arranged two-dimensionally and a desired illuminance distribution can be obtained by selectively lighting LED elements in accordance with work to be inspected.
Japanese Unexamined Patent Publication (Kokai) No. 2000-171403 has described a surface inspection apparatus for inspecting for defects on the flat surface of an article, which provides an even image in a wide image pick-up area by illuminating the surface so that regularly reflected light enters a video camera (an area sensor camera).
On the other hand, few articles have a flat surface and actually, most articles have an almost flat and continuous surface including non-flat parts with small convex shapes and concave shapes. It is demanded that the surface of such an article can be inspected. However, the manner in which light is reflected by a non-flat part with small convex shapes and concave shapes resembles the manner in which light is reflected by a defective part such as a flaw and therefore, a non-flat part is also detected as a defective part. This means that all the article surfaces are judged to have defective parts and a problem arises in that the inspection is no longer effective.
In order to solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 6-249792 and Japanese Unexamined Patent Publication (Kokai) No. 2000-47369 have described a configuration in which a non-flat part can be prevented from being detected as a defective part by providing a mask to a non-flat part of an image to exclude the non-flat part from the target area to be inspected.
Moreover, Japanese Unexamined Patent Publication (Kokai) No. 10-103938 has described a configuration in which a non-flat part is not detected as a defective part by illuminating an article surface from two or more different angles.